The use of passive optical sensors is desired for future military and commercial aircraft, to reduce the negative effects of electromagnetic interference. Many applications exist for passive, optical rotational position sensors (also referred to herein as rotation sensors or angle sensors), but existing sensors are inadequate in terms of resolution, size, and in-line performance monitoring. Many future applications will require optical rotation sensors with 12-bit angular resolution, i.e., 0.09.degree. out of 360.degree. or better. These sensors must be comparable in size and performance to electronic rotary variable differential transformers (RVDTs), which they are designed to replace. An additional shortcoming of current state-of-the-art optical rotation or position sensors is an inability to detect certain anticipated sensor failure modes.
A number of different techniques have been tried in the past to produce optical rotation sensors. One prior art approach is based upon the use of digital encoders and wavelength division multiplexing (WDM). In this approach, a composite optical signal is transmitted to the sensor, the composite signal comprising light in a number of different wavelength ranges. At the sensor, the composite signal is demultiplexed, and the light in each wavelength range is directed to a different track of an encoder plate. Each track includes (for example) transmissive and nontransmissive regions, such that each track encodes one bit of a digital word.
The digital WDM technique has several deficiencies. First, when light emerging from the core of a fiber-optic cable is focused onto an encoder, there is a minimum practical size for the focal spot. This minimum size places a lower limit on the width of the tracks. Practically speaking, this limits the minimum diameter of the encoder plate to above two inches, for a 12-bit rotary encoder using a 100 micron core fiber. Another drawback of the WDM digital encoder is that no adequate and optically simple self-monitoring scheme has been discovered for such a sensor.
Another prior art approach is to fabricate an analog sensor that uses a single analog encoder track having a variable thickness or a variable density dot pattern. The analog track transmits a varying amount of light, depending on the angular position of the encoder plate. Minimum size for this type of sensor is limited by the dimensions of the scanning aperture, and the granularity of the coding or dot pattern.
Other prior approaches have combined analog and digital techniques, for example, as shown in U.S. Pat. No. 4,964,727. This approach overcomes shortcomings of the pure analog or pure digital approaches. However, it is still limited with regard to the minimum encoder plate diameter by feature sizes on the encoder plate, and by dimensions of the scanning aperture.
Another prior approach measures angles by using two linearly polarized beams of light transmitted through a rotating polarizer, the orientation of the polarizer being the angle to be measured. The planes of polarization of the beams are offset 90.degree. from one another. As a result, the intensities of the portions of the beams that are transmitted through the rotating polarizer vary in the manner shown by curves 12 and 14 in FIG. 1. It is assumed that an angle of 0.degree. corresponds to the rotating polarizer having its transmission axis parallel to the plane of polarization of the first beam (curve 12). Curve 12 therefore has maximum values at 0.degree., 180.degree., etc. Since the second beam has its plane of polarization 90.degree. from the first beam, curve 14 has maximum values at 90.degree., 270.degree., etc. The angular position of the rotating polarizer may be estimated by taking the arctangent of the square root of the ratio of the two transmitted intensities.
An important advantage of this polarization approach is that performance, in terms of resolution and accuracy, is no longer dependent on the size of features on the encoder plate. A major disadvantage of this polarization approach is that it only provides accurate results over a limited range of angles. As can be seen in FIG. 1, the slopes of curves 12 and 14 are both very small when the angle is near a multiple of 90.degree.. As a result, near such angles, small amounts of noise in the detected signals translate into large errors in the angle estimate. FIG. 1 also makes it clear that the signals represented by curves 12 and 14 are only capable of providing unambiguous angle measurement over one quadrant, e.g., from 0.degree. to 90.degree.. Two additional binary tracks would be required to resolve quadrant ambiguity and permit measurement over 360.degree..